Bias-Modified Schottky Barrier Height-Dependent Graphene/ReSe2 van der Waals Heterostructures for Excellent Photodetector and NO2 Gas Sensing Applications

Herein, we reported a unique photo device consisting of monolayer graphene and a few-layer rhenium diselenide (ReSe2) heterojunction. The prepared Gr/ReSe2-HS demonstrated an excellent mobility of 380 cm2/Vs, current on/off ratio ~ 104, photoresponsivity (R ~ 74 AW−1 @ 82 mW cm−2), detectivity (D* ~ 1.25 × 1011 Jones), external quantum efficiency (EQE ~ 173%) and rapid photoresponse (rise/fall time ~ 75/3 µs) significantly higher to an individual ReSe2 device (mobility = 36 cm2 V−1s−1, Ion/Ioff ratio = 1.4 × 105–1.8 × 105, R = 11.2 AW−1, D* = 1.02 × 1010, EQE ~ 26.1%, rise/fall time = 2.37/5.03 s). Additionally, gate-bias dependent Schottky barrier height (SBH) estimation for individual ReSe2 (45 meV at Vbg = 40 V) and Gr/ReSe2-HS (9.02 meV at Vbg = 40 V) revealed a low value for the heterostructure, confirming dry transfer technique to be successful in fabricating an interfacial defects-free junction. In addition, HS is fully capable to demonstrate an excellent gas sensing response with rapid response/recovery time (39/126 s for NO2 at 200 ppb) and is operational at room temperature (26.85 °C). The proposed Gr/ReSe2-HS is capable of demonstrating excellent electro-optical, as well as gas sensing, performance simultaneously and, therefore, can be used as a building block to fabricate next-generation photodetectors and gas sensors.


Introduction
Graphene and other two-dimensional (2D) materials, particularly transition metal dichalcogenides (TMDs), have attracted much attention due to their unique electro-optical properties [1][2][3]. TMDs consist of smooth surfaces without any dangling bonds and possess significantly low surface states and trapping defects that cumulatively enable rapid charge speed and suppress charge scattering even in a few nanometer-thick layers [4]. Using these materials, the scientific community is able to develop many proof-of-concept devices such as field effect transistors (FETs) [5][6][7], photodetectors [8][9][10], supercapacitors [5,11], solar cells [12], gas sensors [13], electrochemical sensors [14] and biosensors [15][16][17], etc., for years. Amongst these, photodetectors and gas sensors are of significant interest as these possess the capacity of resolving energy and environmental concerns to a certain level [18]. Interestingly, in contrast to graphene (a zero bandgap material) [19], TMDs have finite bandgap values normally between 0.2 to 3 eV [20] depending upon the choice of material and its layer thickness and were found to be a potential substitute for traditional narrow bandgap materials for many electronic and optoelectronic applications. Moreover, their properties are strongly influenced by the choice of metal contacts (either ohmic or Schottky), energy band alignment, and types of TMDs [21]. Additionally, their electro-optical and gas sensor characteristics can also be modified by the electrostatic backgate voltage as well as channel region doping [22,23].
Furthermore, heterostructures (HS) fabricated on either TMDs or with other lowdimensional electronic materials have been found to demonstrate outstanding optoelectronic and gas sensing performance in comparison to their counterparts [24][25][26]. For instance, PbS quantum dots (PbS-QDs)/MoS 2 heterostructure have shown a tremendously high photoresponsivity (~6 × 10 5 AW −1 ) as compared to only an MoS 2 photodetector. Such remarkable photoresponse properties are associated with strong light absorption characteristics of PbS-QDs [27]. Previously, we have demonstrated ZnO-QDs drop cast over MoS 2 nanosheets to study their electro-optical characteristics. The calculated photoresponsivity was found to be 2 × 10 3 AW −1 [28]. Despite excellent photoresponse, these photodetectors have demonstrated low response speeds (0.1-10 s). This limited performance is ascribed to low response rates as well as the environmental hazard nature (Zn or Pb leaching) of QDs [29]. For the development of next-generation optoelectronic devices, high responsivity and rapid photoresponse is a prerequisite. To circumvent this issue, HS based on graphene and 2D TMDs have been established to improve photoresponse speed as well as photoresponsivity. For instance, the graphene/WS 2 photodetector exhibited a responsivity of~950 AW −1 with a response time of 7.85 s [30]. In another study, graphene/MoTe 2 demonstrated a responsivity of~971 AW −1 with a response time of 78 msec [31]. Such reasonable response speed is attributed to graphene's high carrier transport [32].
Moving onward, 2D-materials-based gas sensors have also been found to be of significant interest. From the literature, the gas sensing response strictly depends upon the surface-to-volume ratio (SVR) of the material [33]. In this perspective, graphene was believed to outperform conventional sensors as its atomically thin layered structure possesses ultimately high SVR [34]. However, in addition to SVR, other factors that can influence gas sensing response are semiconducting properties and the density of available reactive sites for the occurrence of redox reactions [35,36]. Since individual graphene layers have no bandgap, however, stacking graphene to other TMDs can resolve this problem as the semiconducting properties of TMDs can easily be modified by the electrostatic gate bias or exposure to light and therefore, gas sensing response can be modulated/improved. Several 2D materials such as MoS 2 , GaSe, GaS, hBN, WSe 2 , etc. were investigated for gas sensing however, there is not much literature available on graphene-based TMDs heterostructures used as gas sensors [13].
Among various TMDs out there, rhenium diselenide (ReSe 2 ) has been found as an excellent 2D semiconducting material possessing a theoretically measured DFT-based direct narrow bandgap (~0.995 (bulk)-1.239 eV (monolayer)) [37] which is significantly lower than conventional TMDs [38]. Recently, Kim et al. [23] investigated HCl-mediated p-doping of ReSe 2 and reported an improved photoresponsivity of 1.93 × 10 3 AW −1 and photoresponse rise/decay time of 1.4/3.1 ms as compared to undoped ReSe 2 (photoresponsivity = 79.99 AW −1 , rise/decay time = 10.5 ms/291 ms). Bach et al. [39] have studied Gr/ReSe 2 barristor devices, however, with limited photoresponsivity of 42 AW −1 and rise/decay time of 33.9/20.8 ms under a high laser wavelength of 656 nm with a light intensity of 189 mW/cm 2 . However, these reports demonstrated lower photoresponse time. Moreover, no gas-sensing performance was demonstrated in these devices. Therefore, it is of great interest to develop a heterostructure that can demonstrate good electro-optical and gas sensing characteristics simultaneously within a single device.
Herein, we have successfully developed a Gr/ReSe 2 hybrid device that can demonstrate exceptional photodetector and gas sensing performance, simultaneously. In our device, graphene and ReSe 2 flakes work as transport and light absorption layers, respectively. We have drawn a comparative analysis of electro-optical performance between individual ReSe 2 and Gr/ReSe 2 devices. The results indicate that the Gr/ReSe 2 photodetector has considerable photoresponsivity (R~74 AW −1 at 82 mW cm −2 ), detectivity (D *~1 .25 × 10 11 Jones) and a high photoresponse (rise/decay~75/3 µs) as compared to an individual ReSe 2 device (R = 11.2 AW −1 , D* = 1.02 × 10 10 , rise/fall time = 2.37/5.03 s). Moreover, the photocurrent and photoresponsivity were calculated as a function of laser light intensity. Furthermore, Schottky barrier height (SBH) evaluation has revealed Gr/ReSe 2 devices demonstrating low SBH (9.02 meV at V bg = 40 V) which is the reason behind the high electro-optical performance of the HS. Finally, Gr/ReSe 2 -HS was tested for NO 2 gas sensing (20-200 ppb). The exceptional performance of our devices is ascribed to highquality graphene, a suitable choice of ReSe 2 flake, residual-free PDMS stamp supported transfer technique and choice of metal electrodes that eventually reveal low SBH.

Device Fabrication
Here, a vertical heterostructure (HS) composed of mono-layer graphene and few-layer ReSe 2 was prepared over SiO 2 (300 nm)/p + -Si substrate. Figure S1 illustrates step-bystep fabrication detail about HS formation. Briefly, CVD-grown monolayer graphene was transferred over SiO 2 /Si substrate by employing the wet transfer method reported elsewhere [40]. As-transferred graphene layer was then patterned into a rectangular shape (hall bar) using photolithography and an oxygen plasma etching process. During O 2 etching, graphene was treated by power (~50 W) for a few minutes to etch undesired graphene. To make a large pattern around the graphene hall bar, a second photolithography process was carried out after which the defined electrodes were filled by Cr/Au (5/30 nm) deposition. On completion of the deposition, the devices were left in acetone for several hours to accomplish the lift-off process.
On another substrate (SiO 2 (300 nm)/p + -Si), we used the scotch-tape method to mechanically exfoliate the ReSe 2 flake. An optical microscope was used to find a suitable ReSe 2 flake of a few-layers thickness which was later transferred to a pre-patterned graphene hall bar using a PDMS stamp and micromanipulator. In the end, the e-beam lithography process followed by Cr/Au (8/120 nm) deposition and subsequent lift-off in acetone were conducted to make final electrical connections to Gr/ReSe 2 HS. Additionally, a moderate temperature annealing process (200 • C for 4 h) was also carried out in a tube furnace under Ar/H 2 (97.5%/2.5%) gas flow to improve adhesion between metal electrodes and flake surfaces.

Characterization
Raman analysis was examined using micro-Raman (Renishaw, Wotton-under-Edge, UK) for monolayer graphene, few-layer ReSe 2 , and Gr/ReSe 2 HS. A laser wavelength (514 nm) of low-power intensity (511 µW) with a spot size of 0.7 µm was used to avoid any kind of structural deterioration due to the laser heating effect. Furthermore, to realize the exact thickness of graphene and ReSe 2 flakes, atomic force microscopy (AFM; n-Tracer, NanoFocus, Oberhausen, Germany) in tapping mode was used.

Electrical and Electro-Optical Measurement
For electrical measurement, Keithley 2400 and Keithley 6485 K (Keithley Instruments, Inc., Cleveland, OH, USA) were used as source meter and picoammeter, respectively. The complete electrical measurement was performed at room temperature and under vacuum (10 −3 Torr) conditions. Further, to estimate Schottky barrier height (SBH), the device's electrical measurement was achieved in the low-temperature range (30-300 K) under high vacuum (10 −4 to 10 −5 Torr). Moreover, to study electro-optical measurement, the devices were tested using the same systems (Keithley 2400 and Keithley 6485 K) under a vacuum in the dark and under laser light irradiation (532 nm) of varying power intensity (82-820 mW cm −2 ).

NO 2 Gas Sensing Measurement
To further test the Gr/ReSe 2 heterostructure ability to sense NO 2 gas, an experimental setup illustrated in Figure S5 was utilized. The desired concentration was achieved by mixing NO 2 (2%) and N 2 (98%) before injecting them into the chamber. For this purpose, a mass flow controller (MFC) was employed which can control concentration and maintain a total gas flow rate of around 1000 sccm throughout the experiment. The gas is injected inside the chamber in such a manner that it reaches the Gr/ReSe 2 HS-based sensor device within a few seconds. Such an experimental arrangement is very effective as it can detect any environmental change quickly. All the measurement was conducted at room temperature (26.85 • C) under ambient atmospheric conditions. The device was also irradiated with light illumination of 532 nm with a power intensity of 310 mW cm −2 to enhance the gas sensing response of the heterostructure. Figure 1a,b illustrates a schematic diagram and an actual device optical image of graphene/ReSe 2 van der Waals heterostructure, respectively, fabricated at Si/SiO 2 substrate and after Cr/Au contacts deposition via e-beam lithography process (scale bar: 5 µm). Figure S1 represents the device fabrication detail and the various steps involved. In brief, CVD-grown monolayer graphene was first etched into a rectangular bar after which a pristine ReSe 2 flake of appropriate thickness was exfoliated over polydimethylsiloxane (PDMS) stamp, was transferred onto graphene and interacted through van der Waals forces using micromanipulation process. Of note, the photodetector devices based on individual ReSe 2 and graphene/ReSe 2 heterostructures have used the same ReSe 2 flake to avoid any discrepancy while measuring photodetector device performance, as different ReSe 2 flakes could have a different capacity for demonstrating photoresponse.

Results and Discussion
Further, Figure 1c represents a scanning electron microscope (SEM) micrograph (scale bar: 5 µm) which reveals a clear heterostructure without any deformation or contamination during the transfer process (yellow dotted line indicates the boundary of monolayer graphene). To further visualize the uniformity of the material and to evaluate the accurate thickness of the ReSe 2 flake, the atomic force microscopy (AFM) image and corresponding height profile are presented in Figure S2. The AFM scanning reveals ReSe 2 thickness to be around~6.4 nm, approximately nine layers [41].   Further, Figure 1c represents a scanning electron microscope (SEM) micrograph (scale bar: 5 µ m) which reveals a clear heterostructure without any deformation or contamination during the transfer process (yellow dotted line indicates the boundary of monolayer graphene). To further visualize the uniformity of the material and to evaluate the accurate thickness of the ReSe2 flake, the atomic force microscopy (AFM) image and corresponding height profile are presented in Figure S2. The AFM scanning reveals ReSe2 thickness to be around ~6.4 nm, approximately nine layers [41].
Raman analysis of ReSe2 and graphene/ReSe2 heterostructure have revealed several distinct peaks between the ranges 100-300 cm −1 , ascribed to the interlayer vibrational decoupling in ReSe2 (Figure 1d). The prominent peaks related to ReSe2 were observed at 124 and 158 cm −1 . These are associated with in-plane (Eg) and out-of-plane (Ag) vibrational modes, respectively [42]. Furthermore, in the case of graphene/ReSe2 heterostructure, some additional peaks related to monolayer graphene were observed; G-peak positioned at 1580 cm −1 and is related to in-plane phonon mode, and 2D-peak located at ~2700 cm −1 is ascribed to double resonance [39,43].

ReSe2 Device Electrical Performance
To explore the device's performance and to investigate the advantage of graphene in the heterostructure, the device's electrical properties were realized from both pristine Raman analysis of ReSe 2 and graphene/ReSe 2 heterostructure have revealed several distinct peaks between the ranges 100-300 cm −1 , ascribed to the interlayer vibrational decoupling in ReSe 2 ( Figure 1d). The prominent peaks related to ReSe 2 were observed at 124 and 158 cm −1 . These are associated with in-plane (E g ) and out-of-plane (Ag) vibrational modes, respectively [42]. Furthermore, in the case of graphene/ReSe 2 heterostructure, some additional peaks related to monolayer graphene were observed; G-peak positioned at 1580 cm −1 and is related to in-plane phonon mode, and 2D-peak located at~2700 cm −1 is ascribed to double resonance [39,43].

ReSe 2 Device Electrical Performance
To explore the device's performance and to investigate the advantage of graphene in the heterostructure, the device's electrical properties were realized from both pristine ReSe 2 channel and Gr/ReSe 2 heterostructure. Figure 2 illustrates the electrical performance of a few-layer ReSe 2 flake fabricated on a Si/SiO 2 substrate. The transfer characteristics (I ds -V bg ) were studied at V ds = 0.2-1 V and are presented both in linear-, and log-scale as shown in Figure 2a,b, respectively. A bias-dependent increase in on-current (I on ) has been observed on increasing V ds from 0.2 to 1 V which demonstrates usual ReSe 2 transistor characteristics, similar to work [22,44]. The field-effect mobility denoted as µ FE can be evaluated by the following relation: In the above relation, the letters "L" and "W" indicate the length and width of the ReSe 2 channel, dI ds dV bg denotes slope related to transfer characteristics, and C bg (115 aF/µm 2 ) represents gate capacitance [45][46][47]. The calculated mobility for the ReSe 2 transistor was 36 cm 2 /Vs. In addition, the V bg -dependent trend of transconductance (g m (µS) = dI ds dV bg ) of ReSe 2 -based FET was presented (inset: Figure 2a) which demonstrate that the proposed devices possess promising potential of delivering larger gain. Furthermore, to find the suitability of the prepared ReSe 2 transistors for digital applications, the devices must possess a current on/off ratio (I on /I off ) of at least 10 4 [48]. Figure 2b presents log-scale I ds -V bg characteristics and the corresponding calculated I on /I off ratio as the inset. Interestingly, the calculated I on /I off ratio (~1.4 × 10 5 -1.8 × 10 5 ) demonstrates an increasing trend with V ds possibly due to an increase in on-current as observed in I ds -V bg characteristics. The calculated mobility and I on /I off are reasonably higher than the minimum requirement and surpass most of the previously reported TMDs on the Si/SiO 2 substrate. Moving further, the output characteristics (I ds -V ds ) related to the ReSe 2 transistor were evaluated, as presented in linear scale ( Figure 2c) and log-scale (Figure 2d), respectively. Almost linear I ds -V ds characteristics in the low bias (±V ds ) region reveal that the Cr/Au contact established nearly ohmic contact with the ReSe 2 channel with low Schottky barrier height (see Section 3.3), as observed previously [49]. ReSe2 channel and Gr/ReSe2 heterostructure. Figure 2 illustrates the electrical performance of a few-layer ReSe2 flake fabricated on a Si/SiO2 substrate. The transfer characteristics (Ids-Vbg) were studied at Vds = 0.2-1 V and are presented both in linear-, and log-scale as shown in Figure 2a,b, respectively. A bias-dependent increase in on-current (Ion) has been observed on increasing Vds from 0.2 to 1 V which demonstrates usual ReSe2 transistor characteristics, similar to work [22,44]. The field-effect mobility denoted as µ FE can be evaluated by the following relation: In the above relation, the letters "L" and "W" indicate the length and width of the ReSe2 channel, dI ds dV bg denotes slope related to transfer characteristics, and C bg (115 aF/μm 2 ) represents gate capacitance [45][46][47]. The calculated mobility for the ReSe2 transistor was 36 cm 2 /Vs. In addition, the Vbg-dependent trend of transconductance (gm(µ S) = ) of ReSe2-based FET was presented (inset: Figure 2a) which demonstrate that the proposed devices possess promising potential of delivering larger gain. Furthermore, to find the suitability of the prepared ReSe2 transistors for digital applications, the devices must possess a current on/off ratio (Ion/Ioff) of at least 10 4 [48]. Figure 2b presents log-scale Ids-Vbg characteristics and the corresponding calculated Ion/Ioff ratio as the inset. Interestingly, the calculated Ion/Ioff ratio (~1.4 × 10 5 -1.8 × 10 5 ) demonstrates an increasing trend with Vds After the electrical transport measurement of the ReSe 2 transistor, a detailed transport measurement was carried out to evaluate Gr/ReSe 2 heterostructure electro-optical performance. Figure 3a illustrates the transfer characteristics (I ds -V bg ) of Gr/ReSe 2 van der Waals heterostructure both in linear-, and log-scale at V ds = 1 V. It is noteworthy that the heterostructure demonstrates similar transfer characteristics as of pristine ReSe 2 transistor ( Figure 2a); however, a high on-current was observed in the heterostructure as compared to pristine ReSe 2 . This is ascribed to the higher carrier mobility of graphene [24]. The detailed transport characteristics (transfer and output) of pristine graphene are also presented in Figure S3. A charge-neutral point (CNP) also known as a Dirac point (DP) was observed around −8 V at V ds = 0.1 V (Figure S3a), which indicates the graphene is a kind of n-doped graphene [50]. It should be noted here that no intentional doping was performed during the synthesis or transfer process. Therefore, the present monolayer graphene is regarded as pristine graphene. Further, the mobility was calculated using the relation µ = (1/C bg ) (∂σ/∂V bg ), where σ = 1/ρ represents sample conductivity. The measured value of electron mobility for monolayer graphene was around 1350 cm 2 /Vs. In addition, the output characteristics (I ds -V ds ) were also performed ( Figure S3b) which shows a linear relation, revealing the ohmic nature of Cr/Au contact with monolayer graphene. Such remarkable performance of monolayer graphene is the key reason for the high-performing Gr/ReSe 2 heterostructure where we have observed mobility of 380 cm 2 /Vs and an on/off ratio~10 4 ). Here, Gr/ReSe 2 heterostructure was prepared using CVD-grown monolayer graphene over which an exfoliated ReSe 2 flake was transferred. Such heterostructure was also reported previously, however with a limited I on /I off ratio of~10 2 [39]. However, the present work has demonstrated an I on /I off ratio (10 4 ), revealing the potential of the studied heterostructure for switching applications. The limited I on /I off could be ascribed to graphene's semi-metallic nature where the Fermi level of graphene and the related work function varies with bias voltage, resulting in controlled carrier transportation across valence/conduction bands. Moreover, the defects during the growth process of graphene and impurities through the transfer process could also play a significant role in controlling device electro-optical performance [51].

Gr/ReSe2 Photodetector Response
To evaluate the photodetector performance based on Gr/ReSe2 heterostructure, photocurrent measurement as a function of Vds at a fixed Vbg = −20 V and λ = 532 nm was presented in Figure 3b. The measured photocurrent at various incident light intensities is significantly higher than what has been observed under dark conditions, revealing the excellent photoresponse of active charge carriers inside the Gr/ReSe2 heterostructure. Interestingly, a linear relationship between ΔIph and light power intensities has been observed (Figure 3c) which indicates that the larger the light intensity, the higher will be the

Gr/ReSe 2 Photodetector Response
To evaluate the photodetector performance based on Gr/ReSe 2 heterostructure, photocurrent measurement as a function of V ds at a fixed V bg = −20 V and λ = 532 nm was presented in Figure 3b. The measured photocurrent at various incident light intensities is significantly higher than what has been observed under dark conditions, revealing the excellent photoresponse of active charge carriers inside the Gr/ReSe 2 heterostructure. Interestingly, a linear relationship between ∆I ph and light power intensities has been observed (Figure 3c) which indicates that the larger the light intensity, the higher will be the electron-hole pair generation which leads to the generation of high photocurrent in these devices [52]. Further, a cyclic measurement was performed which measured photocurrent for five consecutive cycles without any bias voltage at a power intensity of 310 mW/cm 2 , V ds = 1 V and λ = 532 nm (Figure 3d). In this way, Gr/ReSe 2 heterostructure photoresponse stability and results repeatability was verified. Noteworthy, the devices were measured under vacuum conditions to avoid external oxygen or water molecules device degradation. Moreover, the photoresponse was estimated at V bg = 0 V to remove gate dependency or current contribution. However, the supplied V ds was maintained at 1 V to facilitate drift to charge carriers in the channel region of the Gr/ReSe 2 heterostructure. Furthermore, the photoresponse, i.e., the rise and fall time of the photodetector as a function of time, was estimated as shown in Figure 3e. The rise time (τ rise ) and fall time (τ fall ) of the photodetector was calculated using the following fitting equations [28]: where "I ph (t)" represents time-dependent photocurrent, "I dark " indicate dark current under no light illumination, "t" denotes light switching time, and "A" is equation constant. Equations (2) and (3) were used to estimate the rise and fall time of the Gr/ReSe 2 -based photodetector. The calculated values for rise/fall time were 75/3 µs, significantly higher than most of the studied TMDs-based photodetectors [53][54][55]. We have also evaluated photoresponse characteristics from only the ReSe 2 channel-based photodetector ( Figure S4). The results indicate that Gr/ReSe 2 photodetector has higher photo characteristics as compared to the ReSe 2 -based photodetector. In addition to the response time, several other important photodetector performance parameters such as photoresponsivity (R λ ), external quantum efficiency (EQE%), and detectivity (D * ) were evaluated and are presented in Figures 3f and 4, respectively. The "R λ " is equal to photocurrent produced as a unit of light intensity incident on the effective channel area of the photodetector and is given by the relation [56]: where ∆I Ph = I Ph − I dark is the produced photocurrent, "P" denotes light intensity (82-310 mW/cm 2 ) and "A" represents the device-effective area. The calculated R λ as a function of power intensity is presented in Figure 3f and has values between (50-75 A/W). Responsivity decreases as laser power increases. This trend was fitted by the equation R λ = αP β−1 where α and β are constants whereas P corresponds to optical power. The calculated value of β was around 0.844 at maximum fit with R 2 = 0.9423. The calculated value of R λ for Gr/ReSe 2 photodetector is almost 7 times higher than ReSe 2 photodetector (Responsivity~11.2 AW −1 ). EQE is the number of charge carriers produced per incident photon and mathematically expressed as [22]: where, h, c, and λ are plank's constant, speed of light and wavelength of the incident light, respectively. Interestingly, the EQE value is highly dependent on incident light wavelength, and for a fixed value of wavelength, it depends upon the value of photoresponsivity as other factors are constant. Figure 4 demonstrates a decreasing trend of EQE as a function of laser intensity and follows a similar trend as responsivity. The estimated EQE value was between 117-173% higher than the ReSe 2 photodetector (EQE~26.1%). −1 where and β are constants whereas P corresponds to optical power. The calculated value of β was around 0.844 at maximum fit with R 2 = 0.9423. The calculated value of for Gr/ReSe2 photodetector is almost 7 times higher than ReSe2 photodetector (Responsivity ~11.2 A W −1 ). Detectivity (D * ) is defined as the device's ability to detect signals of a weaker strength. This is mathematically given by the relation [28,57]: D* is described in the unit of Jones, and one Jones = 1 cm Hz 1/2 W −1 and I dark represent current under no illumination. Figure 4 shows D* as a function of light intensity follows a decreasing trend just like responsivity and EQE. It has values between 0.8-1.2 × 10 11 , significantly higher than ReSe 2 photodetector (D*~1.02 × 10 10 Jones). All these results indicate Gr/ReSe 2 van der Waals heterostructure-based photodetector has superior performance compared to only the ReSe 2 material-based photodetector. This means graphene has a governing role in outperforming Gr/ReSe 2 photodetector as it enhances transport rates of photo-carriers produced in ReSe 2 due to the high carrier mobility provided by graphene. The photodetector performance was compared to previously published reports as presented in Table 1.

SBH Estimation
Next, we have estimated Schottky barrier height (SBH) denoted as Φ SBH, and describe it as an energy barrier faced by the electrons while moving across the metal-semiconductor junction. Schottky-Mott's rule was used to predict the value of Φ SBH . It states that the Φ SBH varies proportionally with the difference between the semiconductor's electron affinity and the metal's work function. Interestingly, many semiconductors do not satisfy this rule due to the generation of metal-induced gap states which pin the bandgap close to the Fermi level. Such unwanted effect is regarded as Fermi-level pinning [58]. Therefore, it is highly desirable to select proper metal and semiconductors to minimize SBH value so that devices with ultimate electro-optical performance could be achieved. Here we chose Cr metal (work function~4.5 eV) [59] to deposit as electrodes over the semiconductor (i.e., ReSe 2 devices) and Gr/ReSe 2 heterostructure to define the channel. Temperature-dependent transfer characteristics (I ds -V bg ) for the ReSe 2 transistor and Gr/ReSe 2 heterostructure were determined and presented in Figure 5a,b. The curves were obtained at various temperatures (300, 250, 200, 180, 140, 120, 100, 80, 50 and 30 K). Noteworthy, in the transfer curve, the current values increase as the temperature increases contrary to previous reports which claim a kind of metal-to-insulator transition (MIT) around 200 K [22]. Since the devices prepared in the present work are realized over Si/SiO 2 substrate, which possesses several impurities or defect states, it therefore hinders MIT observation in these devices. From the literature, it has been studied that gate-dependent carrier transport in thin layers of TMDs located near to dielectric substrate is strongly affected by the impurities and various disorders from the dielectric substrate. Therefore, these devices do not demonstrate MIT, which is in agreement with what we have observed in the present study. Moving further, the SBH value was calculated considering standard thermionic emission theory and using the below relationship [60]: where A area represents the device's effective area, A * is Richardson's constant, I ds sourcedrain current through the device channel, V ds indicate source-drain voltage, η is the ideality factor, q represents electron charge, T is temperature and k is the Boltzmann constants. Figure 5c,d illustrates individual ReSe 2 and Gr/ReSe 2 heterostructure device's Richardson plot, i.e., ln (I s /T 2 ) versus q/kT in the reverse bias saturation regime where the obtained data was linearly fitted for each V bg value. Based on the concept of thermionic emission theory, the slope of linearly fitted curves gives the value of Schottky barrier height (Φ SBH ) as presented in Figure 5e,f. Interestingly, the calculated values of Φ SBH are lower/higher at positive/negative V bg values and do not vary linearly with the gate voltage. Moreover, there could exist three different transport regimes based on applied V bg [61]. At low V bg , the device was considered in a switch-off state with the highest value of Φ SBH and the only transport existed due to the thermal agitation of electrons crossing the barrier. Upon increasing V bg , the Φ SBH decreases and the conduction band of ReSe 2 started moving downward resulting in an exponential rise of current as obvious from the transfer characteristics of Figure 2b. Upon further increase in V bg , a flat band condition (V bg = V FB ) reaches which exists in the subthreshold region of transfer characteristics. Moving on, for V bg > V FB , the device underwent a Schottky band regime as obvious by the bent downward part of I ds -V bg characteristics, revealing a combination of thermionic and field emission transport. Finally, with more increase in V bg , there exist a tunneling current through Cr/ReSe 2 barrier which became the major transport mechanism leading to the linear region in I ds -V bg characteristics. As the devices are prepared over Si/SiO 2 substrates, therefore, a lot of charge impurities and surface traps are expected from the substrate surface that could significantly affect the transport mechanism. It is interesting to note that SBH value is significantly lower for Gr/ReSe 2 heterostructure (Φ SBH = 179 − 9 meV for V bg = 0-40 V) as compared to the individual ReSe 2 device (Φ SBH = 274 − 45.4 meV for V bg = 0-40 V). Such low SBH value is dominated by thermionic field emission and could be attributed to the graphene layer which, in the case of Gr/ReSe 2 heterostructure devices, acts as an impurity buffer layer. This will lead to a lesser amount of charge trapping in these devices which is evident from improved electro-optical performance in Gr/ReSe 2 devices as compared to individual ReSe 2 devices.
where represents the device's effective area, A * is Richardson's constant, source-drain current through the device channel, Vds indicate source-drain voltage, is the ideality factor, q represents electron charge, T is temperature and is the Boltzmann constants.

Energy Band Diagram
To further understand, we have presented the energy band diagram of the Gr/ReSe 2 heterostructure as shown in Figure 6a,b. Interestingly, the substrate impurities induce p-type doping of monolayer graphene leading to an increased density of holes within the graphene layer which, in turn, shift the Fermi level lower as compared to what was observed in the case of pristine graphene. Furthermore, from transfer characteristics (Figure 2), ReSe 2 appears to be an n-type semiconductor; therefore, its Fermi level will be situated close to the conduction band. Moving further, as a result of ReSe 2 transferred over CVD-grown monolayer graphene, a band bending occurs across the valence/conduction bands of ReSe 2 to align the Fermi levels of both materials. This band bending is attributed to the work function difference between graphene and ReSe 2 . Upon biasing Gr/ReSe 2 heterojunction, two types of band diagrams are possibly manifested in Figure 6b,c. It should be noted that the graphene layer is in direct contact with Si/SiO 2 (300 nm) dielectric substrate; therefore, an externally applied electric field could significantly modify its Fermi level and thus the associated work function [62]. To calculate the Φ SBH between graphene and ReSe 2 , a difference between graphene Fermi level and electron affinity of ReSe 2 was obtained, i.e., Φ SBH = Φ Gr − χ ReSe2 . From this relation, one can understand that Φ SBH is strictly dependent upon Φ Gr and can be modified if an external voltage is applied across graphene as it changes its work function in the heterostructure device. Figure 6b explains the band diagram under V bg < 0 bias condition. In this state, graphene became more holedoped as is obvious from the downward shift of the graphene Fermi level which eventually increases its work function and the Φ SBH . The value of Φ SBH of Gr/ReSe 2 heterostructure keeps on increasing with an increase in negative V bg . The highest value of Φ SBH was observed at~300 meV at V bg = −40 V. Moving further, in the case of V bg > 0, the electrons are generated in the graphene layer (Figure 6c). Under this condition, the Fermi level of graphene shifts in the upward direction resulting in a reduced Φ SBH value. The lower value of Φ SBH under forward biasing (V bg > 0) facilitates easy transport of majority carriers across the junctions thus an increase in on-current was realized in the transfer characteristics ( Figure 3a). As our heterostructure is composed of atomically thin flakes (i.e., monolayer graphene and few-layer ReSe 2 ), the possibility of incomplete electric field screening in both materials cannot be evaded. Thus, both components of the heterostructure are affected by the electric field modulation, as evident in previous reports [63]. The estimated values provide an accurate assessment of Φ SBH at the Gr/ReSe 2 van der Waals interface via electric field modulation. Since the Φ SBH demonstrates a strong gate modulation, one can speculate that the electric field-induced transport mechanism is the governing mechanism in the devices demonstrated here. accurate assessment of ΦSBH at the Gr/ReSe2 van der Waals interface via electric field modulation. Since the ΦSBH demonstrates a strong gate modulation, one can speculate that the electric field-induced transport mechanism is the governing mechanism in the devices demonstrated here.

Gr/ReSe2 Heterostructure as NO2 Gas Sensor
To demonstrate the NO2 gas sensing experiment, the prepared devices were placed in a mass flow controller (MFC) setup as illustrated in Figure S5. Individual Gr, ReSe2, and Gr/ReSe2 heterostructure devices were evaluated at room temperature under different gas concentrations (20-200 ppb). During the gas sensing experiment, the samples were continuously irradiated by a light illumination of 532 nm as it improves gas sensing response [26]. Additionally, the samples were irradiated by visible light instead of UV to avoid any damage from the light source. Figure 7a illustrates the gas sensing dynamic response of Gr/ReSe2 heterostructure under various NO2 concentrations at Vds = 1 V and incident light illumination of 532 nm with the intensity of 310 mW cm −2 . The gas sensing response was determined by the following relation: where "Rg" and "Ra" indicate device resistance under NO2 gas environment and in air. Noteworthy, since two-dimensional materials (2D) bestow a large surface-to-volume ratio, their heterostructure could demonstrate a relatively high NO2 gas sensing response despite being under low NO2 concentration. It is obvious from Figure 7a that the Gr/ReSe2 heterostructure demonstrates a monotonically increasing gas sensing response with rising NO2 concentration from 20 to 200 ppb. Compared to previous reports on 2D materialsbased gas sensors, our heterostructure demonstrates a large response of ~36% even at a low NO2 concentration of 20 ppb [64][65][66]. Moving further, we have explored our hetero- Figure 6. (a) Band diagram of graphene and ReSe 2 before contact, (b) after contact when V g < 0, and (c) after contact when V g > 0.

Gr/ReSe 2 Heterostructure as NO 2 Gas Sensor
To demonstrate the NO 2 gas sensing experiment, the prepared devices were placed in a mass flow controller (MFC) setup as illustrated in Figure S5. Individual Gr, ReSe 2, and Gr/ReSe 2 heterostructure devices were evaluated at room temperature under different gas concentrations (20-200 ppb). During the gas sensing experiment, the samples were continuously irradiated by a light illumination of 532 nm as it improves gas sensing response [26]. Additionally, the samples were irradiated by visible light instead of UV to avoid any damage from the light source. Figure 7a illustrates the gas sensing dynamic response of Gr/ReSe 2 heterostructure under various NO 2 concentrations at V ds = 1 V and incident light illumination of 532 nm with the intensity of 310 mW cm −2 . The gas sensing response was determined by the following relation: where "R g " and "R a " indicate device resistance under NO 2 gas environment and in air. Noteworthy, since two-dimensional materials (2D) bestow a large surface-to-volume ratio, their heterostructure could demonstrate a relatively high NO 2 gas sensing response despite being under low NO 2 concentration. It is obvious from Figure 7a that the Gr/ReSe 2 heterostructure demonstrates a monotonically increasing gas sensing response with rising NO 2 concentration from 20 to 200 ppb. Compared to previous reports on 2D materialsbased gas sensors, our heterostructure demonstrates a large response of~36% even at a low NO 2 concentration of 20 ppb [64][65][66]. Moving further, we have explored our heterostructure gas sensing response for various light intensities. Under NO 2 gas flow (200 ppb) and light wavelength (532 nm) exposure, the gas sensing response of heterostructure was evaluated with increasing light intensities as illustrated in Figure 7b. The gas sensing response rises from~10% to~200% as the light intensity increases from 0 to 310 mW cm −2 . This is ascribed to the fact that more electrons are produced by increasing light intensity and made their way from the heterojunction to the NO 2 , subsequently leading to improved gas sensing response. Here, it is also noted that only Gr (black curve), and individual ReSe 2 (blue curve) devices have demonstrated lower gas sensing performance of about 20%, and~41% as compared to Gr/ReSe 2 heterostructure (~200%; red curve) under similar conditions as illustrated in Figure S6. This is attributed to enhanced electronhole pairs generation at heterojunction interface under light exposure and agrees well with previous reports [67,68]. To further evaluate the NO 2 gas response efficiency of the prepared Gr/ReSe 2 heterostructure, the transient response was determined for 200 ppb NO 2 concentration and under light illumination (532 nm) with intensity 310 mW cm −2 as displayed in Figure S7. As-calculated room temperature NO 2 response/recovery time for the heterostructure was found to be 39/126 sec which is comparable with the top gas sensors based on 2D materials so far [69,70]. Next, the Gr/ReSe 2 heterostructure was tested for gas sensing stability as illustrated in Figure 7c. The freshly prepared heterostructure (0-day) was placed under NO 2 (200 ppb) and light wavelength (532 nm) with an intensity of 310 mW cm −2 exposure. The resultant NO 2 gas sensing response was around~200%. The device was tested again after a month under ambient conditions. The resultant gas sensing response was~180% which is only 20% less than the original value, revealing the highly stable nature of Gr/ReSe 2 heterojunction. Furthermore, to see the stable working potential of prepared sensor, relative humidity effect on response factor was tested as demonstrated in Figure S8. The results reveal similar sensing response under various humidity conditions, i.e., relative humidity (RH: 20-80%). The minor degradation was observed for RH = 20% and RH = 80%, however, no significant change was observed for RH: 40%, 60%, which is typical working conditions in most of the laborites. The results indicate that humidity is not the main factor for consideration to demonstrate consistent NO 2 gas sensing behavior. Other parameters, such as NO 2 gas exposure, light wavelength and intensity and exposure duration are the main factors that influence sensor properties. concentration and under light illumination (532 nm) with intensity 310 mW cm −2 as displayed in Figure S7. As-calculated room temperature NO2 response/recovery time for the heterostructure was found to be 39/126 sec which is comparable with the top gas sensors based on 2D materials so far [69,70]. Next, the Gr/ReSe2 heterostructure was tested for gas sensing stability as illustrated in Figure 7c. The freshly prepared heterostructure (0-day) was placed under NO2 (200 ppb) and light wavelength (532 nm) with an intensity of 310 mW cm −2 exposure. The resultant NO2 gas sensing response was around ~200 %. The device was tested again after a month under ambient conditions. The resultant gas sensing response was ~180% which is only 20% less than the original value, revealing the highly stable nature of Gr/ReSe2 heterojunction. Furthermore, to see the stable working potential of prepared sensor, relative humidity effect on response factor was tested as demonstrated in Figure S8. The results reveal similar sensing response under various humidity conditions, i.e., relative humidity (RH: 20-80%). The minor degradation was observed for RH = 20% and RH = 80%, however, no significant change was observed for RH: 40%, 60%, which is typical working conditions in most of the laborites. The results indicate that humidity is not the main factor for consideration to demonstrate consistent NO2 gas sensing behavior. Other parameters, such as NO2 gas exposure, light wavelength and intensity and exposure duration are the main factors that influence sensor properties.

Conclusions
We have successfully fabricated Gr/ReSe 2 van der Waals heterostructure (vdW-HS) using CVD-grown monolayer graphene (patterned into a rectangular bar), mechanically exfoliated few-layer ReSe 2 and all-dry PDMS stamp-assisted transfer method. The prepared HS has been used to evaluate electro-optical properties and gas sensing performance. By exploiting narrow bandgap features of ReSe 2 , the prepared Gr/ReSe 2 -HS demonstrated an excellent mobility of 380 cm 2 /Vs, current on/off ratio~10 4 , photoresponsivity (R~74 AW −1 @ 82 mW cm −2 ), detectivity (D *~1 .25 × 10 11 Jones), external quantum efficiency (EQE~173%) and rapid photoresponse (rise/fall time~75/3 µs) as compared to individual ReSe 2 device (mobility = 36 cm 2 V −1 s −1 , I on /I off ratio = 1.4 × 10 5 -1.8 × 10 5 , R = 11.2 AW −1 , D* = 1.02 × 10 10 , EQE~26.1%, rise/fall time = 2.37/5.03 s). Such remarkable performance is due to the combined result of strong light absorption of ReSe 2 and high carrier transport of graphene. Moreover, low value of Schottky barrier height (SBH) for Gr/ReSe 2 -HS (9.02 meV @ V bg = 40 V) confirms that graphene is somehow working as defects (due to Si/SiO 2 dielectric substrate) suppressing layer. Furthermore, the HS was subjected to NO 2 gas environment under various humidity conditions to test its aptitude for the gas sensor at room temperature (26.85 • C). The results demonstrated a high response, good reversibility, and gas selectivity under light irradiation of 532 nm. Interestingly, the proposed HS has illustrated an excellent response even toward low ppb-level NO 2 exposure (20 ppb), revealing the proposed HS is superior to most of the reported literature. To conclude, our Gr/ReSe 2 -HS is capable of demonstrating excellent electro-optical as well as gas sensing performance simultaneously and, therefore, can be used as a building block to fabricate next-generation photodetectors and gas sensors to further enhance optoelectronics research domain and internet of things (IoT) devices. Moreover, it offers a potential sensing platform for cost-effective environmental monitoring systems.